Ancient German history - Pre-Roman Invasion era (up to 476-ish)
Pre-Crusade history (476-1066-ish)
I also like learning about Russian history, like.. pre-Mongol Invasion stuff
And English history.. pre-Norman.
Basically, any history that happened before a big modification to a culture. Pre-Xia-dynasty in China, before emperors were succeeded by sons (they were elected by elders).
And I can tackle the language question a bit more...
English and Frisian (a DUTCH dialect near the coast) are very closely related. I'm too lazy to find conjugates and such... but.. yeah... why though?
After the Romans retreated from Britain in around 400AD, they left behind Romanized Celts, and Picts/Scots/Irish. These Celts formed little kingdoms (hey, Ireland had 4-5 kingdoms). However, to the east, a bunch of people began moving. Saxons from Gotland, near Sweden (the word Goth I believe has origins in this), Angles from modern-day Netherlands, and Jutes from Denmark all moved to England. This is where the whole story of Beowulf, and King Arthur, come in. Beowulf was about the Gotlanders (Saxons, not from Saxony though). Arthur was about the Romanized Celts (Arthur was a Romano-Celt). So you have a bunch of fighting in where modern-day England is. Eventually, the Celts moved to Wales or Brittany where they carved out kingdoms. In England, you had your Anglo-Saxon kingdoms. This is where modern English came from. Old English is from the languages of the Angles and the Saxons. The word "good" sounds like the German word "gut" which means good. Essentially, the ancestors of modern English were from Germany and Sweden and Netherlands.
More interesting facts: The middle of England was a Viking kingdom until the Normans came about.
English sounds different than other Germanic languages because of its isolated position, and its earlier branching off from the main Germanic tongues.
The reason why English has Latin and French words is because of Norman invasions (which brought French into mainstream) and Christianity (the Roman Church) which had Latin words. The words "cow" and "beef" are examples. Beef is from a word which evolved into the modern French word "boeuf" which essentially means "cow". We eat beef, but we raise cows. We eat poultry but we raise chickens (poulet). Strangely, we also say "we're having chicken". I guess chickens were less popular in Normandy...